Loopback testing is a well known technique for examining the operational characteristics of communication systems. In a loopback test, a message is transmitted from a source and traverses some portion of a communication system, ultimately returning to the source. Upon the return of the message, the information content of the message and/or the physical attributes of the message (e.g., signal strength, signal-to-noise ratio, and other parameters of interest) can be observed and compared to the same information and/or parameters as they existed when the message was initially transmitted. Information about the state of the portions of the communication system that the message traversed and indications of the quality of service that is being provided can be extracted from comparing the original message and the message upon its return.
In some communication systems, setting up a loopback test requires either or both special equipment and a disruption of the normal service provided by the communication system. In some instances, for example in certain packet-based communication systems, loopback testing can involve rather sophisticated handling of the test message. In some communication systems, loopback testing can be disrupted by the normal operation of the communication system itself.
In Ethernet networks it is common to have control protocols such as Spanning Tree. Spanning Tree includes a mechanism to detect network loops. If a loop is detected, the network monitoring software (NMS) will disable the ports on which the loop was detected, as loops waste valuable bandwidth. This means that if an apparatus, such as a test instrument, is connected that reflects frames back to the transmitter, in all likelihood, the port on which the reflector is connected will be disabled. Other protocols exist that also detect loops.
There is a need for loopback testing apparatus and methods that operate in a convenient and user-friendly manner and that can perform the desired tests without disruption by the protocols of the communication system.